This invention relates to detectors in an FM stereo receiver for detecting a tone in a multiplexed stereo radio signal, and more particularly for detection of an FMX indicator tone and then for switching the operating mode of the receiver from standard stereo mode to FMX stereo mode.
"FMX" and "FMX Stereo" are TRADEMARKS of Broadcast Technology Partners, Bloomfield Hills, Mich. FMX broadcasting of stereo FM signals entails broadcasting the conventional FM signal modulated with a multiplexed stereo composite signal that includes the standard low frequency M-band containing the L+R audio signal, the standard 19 Khz pilot signal, and the standard S-band containing the AM modulated L-R audio signal with a suppressed 38 Khz/0.degree. subcarrier. The broadcast FMX composite signal additionally includes within the frequency spectrum of the standard S-band, a companded (amplitude compressed) version of the L-R, signal herein referenced to as an S'-band, having a suppressed phase-shifted subcarrier of 38 Khz/90.degree.. The angle is taken relative to the phase (zero crossings) of the 19 Khz pilot.
This FMX transmission is compatible with standard FM stereo receivers that will be capable of providing stereo reception of the FMX broadcast signal but without the FMX reception advantage. An FMX receiver, however, will be capable of receiving that FMX transmitted signal over a greater physical range. An FMX receiver includes a composite-signal decoding circuit with a 38 Khz/0.degree. subcarrier generator and a multiplier to produce the conventional L-R (the left audio minus the right audio) signal, but in addition includes a decoding circuit with a 38 Khz/90.degree. subcarrier generator and a multiplier to produce a second L-R audio frequency signal. That second L-R audio signal that was companded and transmitted modulated on the subcarrier that is 90.degree. out of phase with the conventional subcarrier carrying the L-R signal, is then passed through an expander circuit in the receiver to restore it to its original condition as it was before being companded in the transmitter. The expanded L-R S'-band and S-band signals are demodulated then summed and combined with the L+R M-band in a standard matrixing circuit in the receiver for producing separate left audio and right audio signals, as is more fully explained by Torick et al in U.S. Pat No. 4,485,483 issued Nov. 27, 1984 and now assigned to Broadcast Technology Partners. An FMX transmitter and receiver are shown respectively in FIG. 1 and FIG. 2.
When an FMX-capable receiver is tuned to a non-FMX stereo transmission, it is necessary to disconnect the second L-R audio signal (demodulated from the S'-band) to avoid noise and distortion. The suppressed subcarrier 38 Khz/90.degree. of the S'-band is AM-modulated with a tone of subaudible frequency to serve as an FMX indicator tone that can be detected in the receiver and used in the receiver to switch in the S'-band decoder circuits and thus enable FMX-mode receiver operation. The detection of the tone also is used to turn on a lamp indicating to the listener that an FMX-type signal is being received. A tone frequency of about 10 Hz is usually chosen.
A tone detector circuit used in receivers of the prior art is illustrated in FIG. 2. This circuit is basically the same phase-locked loop circuit as has been employed in conventional FM stereo radios to detect the 19 Khz pilot.
Tone detector circuit 10 of FIG. 2 has an input conductor 12 typically connected to the output of a 38 Khz/90.degree. demodulator 14 having an input connected to the composite signal conductor 13 at the output of the discriminator of the FMX receiver. This FMX composite signal includes the 10 Hz FMX indicator-tone. Referring to the prior art tone detector 10 of FIG. 3, the demodulator signal is passed through a three-pole low pass filter 15 having a bandpass at 3 db down of about 100 Hz or lower that removes almost all but the 10 Hz tone indicator signal. The output of filter 15 is connected to a mixer 16 that is in turn connected to a level detector 18 via a low pass filter 20. When the FMX tone is detected, the output of the level detector 18 changes from one state or level to another indicating at conductor 17 the reception of an FMX signal transmission. The output of filter 15 is also connected to the phase-locked loop reference signal generator 11 that includes a mixer 22, a single pole low pass filter 24, an oscillator circuit 26 that locks up at a multiple (e.g. 32) of the 10 Hz tone signal, and a frequency divider 28 that divides the 25 oscillator frequency by the same multiple factor (e.g. 32).
Filter 15 removes most of the quadrature L-R audio from the signal presented at the input 12. The low pass filter 20 removes or greatly reduces signal components above about 10 Hz. The voltage controlled oscillator (VCO) 26 is phase-locked to a multiple of the tone signal and the frequency divider 28 divides by the same multiple and provides at output 30 a 10 Hz/0.degree. signal in phase with the incoming 10 Hz tone component of the phase-shifted (quadrature) L-R audio signal (S') at input 12, while at output 32 the quadrature-phase shifted 10 Hz/90.degree. signal is produced for closing the phase-locked loop. The level detector output at conductor 17 is the tone indicator signal, and is also the mode-switch control signal.
Alternative to the phase-locked loop tone detector of FIG. 3 is a detector (not shown) having at least one multipole low pass filter and a counter for dividing down from the 19 Khz pilot signal by 1920 to generate the 10 Hz FMX indicator signal.
The phase-locked loop tone indicators of the prior art have a long response time approaching 2 seconds, have an oscillator requiring factory adjustment (e.g. via variable resistor 34), and have no means to cancel out the 10 Hz signal before or at the FMX expander. The 10 Hz signal in the expander tends to produce unwanted modulation products in the lower part of the audio spectrum. Thus there is required a multipole filter 15 preceeding the phase-locked loop to prevent lockup on odd harmonics of the oscillator, and leads to the need for a number of discrete components and requires a corresponding number of IC pads for connection of the discrete components to the integrated circuit that preferably contains most of the tone indicator circuitry. A substantial part of the cost of the prior art tone detector is attributable to the later two features.
It is an object of the present invention to provide an FMX radio transmission and recovery system having a pilot-coherent FMX-transmission tone indicator signal carried in the transmitted composite signal, and a receiver having a tone detector with a fast detection response time.
It is another object of the present invention to provide a low cost tone detector for use in an FMX-capable stereo radio receiver for cancelling the subaudio FMX-mode-indicating tone in the L-R expander circuit of the receiver.